1. Field of the Invention
The present invention relates to a process and an apparatus of producing an optical disk (or an optical medium), and particularly a laminate type optical disk, and also relates to a process of producing a substrate to be laminated for the production of the optical disk. For example, such an optical disk includes those for DVD (Digital Video Disk) or the like.
2. Related Art
A process has been known in which a single optical disk for DVD is produced by laminating a plurality (usually two) of substrates each having predetermined irregularities for a recording layer on its one main surface so that the single optical disk has a plurality of the recording layers. Such a process and an apparatus therefor are disclosed in for example WO97/35720, parts of which are incorporated herein by this reference provided that they are relevant to the present invention described hereinafter.
Concretely, the optical disk is produced by the following process comprising the steps of:
(1) preparing a pair of same or different two substrates each of which has been injection molded of a transparent resin so as to provide with fine irregularities on its one main surface and each of which has a metal (gold, aluminum or the like) thin film on said one main surface (usually, each substrate is of a circular form having a center hole);
(2) supporting the substrates while keeping a narrow gap between them so that the metal layers on the substrates are facing to each other;
(3) supplying an adhesive such as a UV curable adhesive into the gap and spreading it throughout the gap so that a laminate of the pair of the substrate and the adhesive between them is obtained; and
(4) bonding the pair of the substrates together by curing the adhesive in order to join the substrates. For example, by irradiating UV on the laminate, the UV curable adhesive is cured.
Reading data (or information) from an optical disk or writing data into the optical disk is carried out by irradiating a laser beam having a predetermined wavelength onto the optical disk while the beam traversing the optical disk from its innermost or outermost periphery so that the beam passes through the transparent substrate and reaches a recording layer while the disk is being rotated at a high speed. Since a density of data which is to be recorded (or written in) or which has been recorded beforehand in the recording layer is extremely high, it is important that the laser beam is accurately focused onto a predetermined position of the optical disk. If flatness of the disk as a whole is not good, the laser beam cannot be focused onto the predetermined position of the optical disk so that data is likely to be incorrectly read or recorded.
Upon the production of the optical disk, it is therefore desirable that no deformation such as warpage or strain is observed in the optical disk as a final product, and also that occurrence of such deformation is suppressed after the production (i.e. during storing and/or using the optical disk), so that the produced optical disk has and keeps the improved flatness.
Considering the deformation which occurs in the optical disk, there comes one following possibility of a mechanism which causes the deformation:
The deformation of the optical disk is resulted from deformation with time of the substrate made of the transparent resin. Particularly, the substrate just after the molding which is in a dry condition at a considerably high temperature is likely to absorb moisture from its surrounding atmosphere, so that the substrate deforms as it absorbs the moisture.
Then, it has been proposed to subject the substrate to a treatment in which the molded substrate is left over an extended period to stabilize a moisture content of the substrate, and then subject to the metal film formation step as a subsequent step in which a metal thin film is deposited onto the substrate. The treatment may be also referred to as an xe2x80x9cannealing treatmentxe2x80x9d, and generally it is carried out over a period between few hours and 24 hours.
However, it is very ineffective to include the annealing treatment requiring the extended period in an integrated production line when mass production of the optical disk is intended. In order to carry out the annealing treatment, a large space is required for temporarily storing a large number of the molded substrates. If conditions of an atmosphere for the annealing treatment are changed due to some reason, an extent of the effect derived from the annealing treatment varies, which results in the various substrates which have different moisture contents. Further, it takes a considerably long time to produce the optical disk as a final product since starting the molding of the substrate.
In addition, there is further warpage (or warp) which appears with time after completing the production of the optical disk as well as further warpage which appears depending on application conditions of the optical disk. Even though the produced optical disk has passed product testing so that it has been proved to have a warpage within a predetermined specification, the warpage of the optical disk sometimes increases while it is repeatedly used or stored for an extended period, which may adversely affect recording the data into or reading the data from the optical disk. This would be because an internal stress or strain contained in the optical disk during its production is released with time or due to its application conditions so that the warpage additionally occurs.
Considering the above related art, it is therefore an object of the present invention to suppress the occurrence of the warpage upon the production of the optical disk and the warpage of the optical disk with time so that the optical disk of the laminate type is efficiently produced which has better flatness.
In the first aspect, the present invention provides a process of producing a laminate type optical disk comprising the steps of:
(1 ) molding a pair of substrates of a transparent resin at least one of which substrates has irregularities on its one main surface;
(2) forming a metal thin film on said one main surface having the irregularities, and then
(3) laminating the pair of the substrates while the metal thin film is located inside and bonding the substrates by means of an adhesive which is supplied between the substrates,
the process being characterized in that after the step (1), a waiting treatment is carried out in which the molded substrates are cooled to such a temperature that a difference between said temperature and a temperature of an atmosphere surrounding the substrates is not greater than 5xc2x0 C. while keeping a moisture (or water) content of the substrates not larger than 0.1 % by weight, and then the step (2) is carried out.
In the present invention, at least one of the pair of the substrates has the fine irregularities on one of its main surfaces. That is, only one substrate may have the irregularities on its main surface, but generally each of the substrates has the irregularities on its main surface. When the optical disk is of a read only memory (ROM) type, the irregularities correspond to pits of recorded data. When the optical disk is of a random access memory (RAM) type or a recordable (or write once) (R) type, the irregularities correspond to lands and grooves. In any type of the optical disks, irregularities which correspond to format information inherently required for the optical disk may be included in said one main surface of either or both of the substrates.
The metal thin film is a layer formed on said one main surface of the substrate having the irregularities which layer reflects light, and the film may be semi-permeable to light (i.e. translucent). For example, the metal thin film is formed by sputtering gold or aluminum. The metal thin film functions as a reflecting layer which reflects a laser beam irradiated onto the optical disk. When the both substrates have the irregularities, the metal thin film is formed on said one main surface of each of the substrates which has the irregularities.
In the case of the optical disk to which the laser beam is irradiated from one side thereof, the metal thin film is semitransparent which is formed on one of the substrates. In the case of the optical disk to which the laser beam is irradiated from both sides thereof, the metal thin film may be impermeable to light (i.e. opaque) which is formed on each of the substrates. When only one substrate includes the irregularities, it is sufficient that said one substrate has the metal thin film on its main surface, and the other substrate does not have to have a metal thin film.
For the read only memory type optical disk, the metal thin film in combination with the irregularities of the substrate reflects the irradiated laser beam depending on the data which was recorded beforehand. Therefore, the metal thin film may be referred to as a recording layer in the case of the read only memory type optical disk.
Also, as well known, for the random access memory type optical disk or the recordable type optical disk, using the irradiated laser beam, data to be recorded is recorded by means of the metal thin film (together with the irregularities on the substrate and a reversibly-changeable or irreversibly-changeable layer which is separately formed on the irregularities of the substrate). Further, using the irradiated laser beam, data previously recorded is read by means of the metal thin film (together with the irregularities on the substrate and the reversibly-changed or irreversibly-changed layer which is separately formed on the irregularities of the substrate). As described above, within the random access memory type optical disk or the recordable type optical disk, the irregularities, the reversibly-changeable or irreversibly-changeable layer and the metal thin film relate to functions of the optical disk of recording and reading the data, and the reversibly-changeable or irreversibly-changeable layer mainly contributes to the functions. Therefore, the reversibly-changeable or irreversibly-changeable layer may be referred to as a recording layer.
Thus, in the case of the random access memory type optical disk or the recordable type optical disk, the reversibly-changeable or irreversibly-changeable layer is formed on said one main surface of the substrate having the irregularities, and the metal thin film is formed on such a layer. Therefore, when the random access memory type optical disk or the recordable type optical disk is produced in the process of producing the optical disk according to the present invention, an additional step which forms the reversibly-changeable or irreversibly-changeable layer is carried out after the waiting treatment and before performing the step (2). In other words, the step (2) which forms the metal thin film comprises forming the reverisbly-changeable or irreversibly-changeable layer on said one main surface of the substrate having the irregularities before the formation of the metal thin film, and then forming the metal thin film on the such layer. For the read only memory type optical disk, such additional step is of course unnecessary.
In the present specification, the term xe2x80x9crecording layerxe2x80x9d is used in the meaning as described above, and thus an element which corresponds to the meaning of the recording layer is different depending on the type of the optical disk. The present invention will be explained hereinafter in detail mainly with reference to the read only memory type optical disk. For the random access memory type optical disk or the recordable type optical disk, there is no substantial difference from the read only memory optical disk except that the reversibly-changeable or irreversibly-changeable layer is formed before forming the metal thin film. The formation of the reversibly-changeable or irreversibly-changeable layer can be carried out in the conventional manner.
In one embodiment, the waiting treatment of the present invention is carried out while keeping the moisture content of the substrates preferably not larger than 0.08% by weight and more preferably not larger than 0.05% by weight. The moisture content herein used is a weight basis percentage of water contained by the substrate relatively to the dried substrate, and it is so-called dry weight basis water content. In another embodiment, the waiting treatment of the present invention cools the molded substrates so that the difference between the substrates temperature and the temperature of the atmosphere surrounding the substrates (which is hereinafter sometimes referred to as merely xe2x80x9ctemperature difference of the substratexe2x80x9d) is not greater than 5xc2x0 C. and preferably not greater than 3xc2x0 C. In a more preferable embodiment, the waiting treatment is so carried out that the temperature difference of the substrate is not greater than 5xc2x0 C. while keeping the moisture content not larger than 0.05% by weight, and such treatment may be carried out for example by leaving the substrates removed from a molding machine for about 1 to 6 minutes to an atmosphere of which temperature is in the range between 20xc2x0 C. and 35xc2x0 C. and of which relative humidity is between 10% and 60%.
It is noted that the xe2x80x9catmospherexe2x80x9d of the present invention is intended to mean a space in which the substrates are subjected to the waiting treatment, and such a space is usually kept at a predetermined temperature and a predetermined humidity (for example at a temperature of 25xc2x0 C. and a relative humidity of 30%). The humidity as described above may be measured in any proper manner, and for example by measuring a weight change of the substrate while it is positioned in the predetermined atmosphere.
In the second aspect, the present invention provides a process of producing an optical disk by laminating a first substrate formed by a first molding machine and a second substrate formed by a second molding machine, which process comprising the steps of:
(1) molding a pair of the first substrate and the second substrate of a transparent resin by means of the first molding machine and the second molding machine respectively, wherein at least one of the substrates has irregularities on its one main surface;
(2) forming a metal thin film on said one main surface having the irregularities, and then
(3) laminating the pair of the substrates while the metal thin film is located inside and bonding the substrates by means of an adhesive which is supplied between the substrates,
the process being characterized in that a difference in a mold temperature between the first molding machine and the second molding machine (which difference is hereinafter sometimes referred to as xe2x80x9cmold temperature differencexe2x80x9d) is established in the range between 2xc2x0 C. and 6xc2x0 C. In the production process of the second aspect, it is preferable to carry out the waiting treatment according to the first aspect of the present invention.
The mold temperature used herein is intended to mean an arithmetic mean of temperatures of a pair of mold halves which together define a cavity (or an inner space) correspond to the substrate when a molten transparent resin is injected into the cavity of the mold which is constituted by the mold halves so as to produce the substrate. Usually, in the laminate type optical disk, one of the pair of the substrates has a rib (or a stack rib) and the other has no rib, and it is preferable that the mold temperature of the molding machine which forms the substrate having the rib is set higher.
In the third aspect, the present invention provides a process of producing a substrate for an optical disk by injection molding wherein a molten transparent resin is injected into a cavity of a mold, the process being characterized in that a difference in temperature between one and the other the pair of the mold halves (which difference is hereinafter sometimes referred to as xe2x80x9cmold half temperature differencexe2x80x9d) which together constitute the cavity is in the range between 0xc2x0 C. and 6xc2x0 C. The substrate produced by this process is preferably used in the production process of the optical disk according to the first aspect or the second aspect of the present invention.
In the fourth aspect, the present invention provides an apparatus of producing an optical disk by laminating a pair of substrates using an adhesive which apparatus comprises
(a) a substrate forming unit which molds the pair of the substrates of a transparent resin, at least one of which substrates has irregularities on its one main surface;
(b) a film forming unit which forms a metal thin film on said one main surface having the irregularities by means of a thin film forming means; and
(c) a bonding unit which joins the pair of the substrates by means of the adhesive,
the apparatus being characterized in that the substrates are passed from the substrate forming unit through the film forming unit to the bonding unit by means of a transporting means, and that the apparatus comprises a waiting treatment means which temporarily holds the substrates in an atmosphere for a predetermined period between the substrate forming unit and the film forming unit. The waiting treatment means performs the waiting treatment as described above.
In the present invention, the production process of the optical disk or the substrate (including forming the substrates, forming the metal thin film and bonding the substrates) can be carried out in the conventionally known manners as described below excepting the waiting treatment, the substrate formation under the specified mold temperature difference and the substrate formation under the specified mold half temperature difference.